Disorders of breathing during sleep are now known to constitute a major health problem throughout the world. Obstructive sleep apnea is an extremely common disease which manifests itself in variable degrees of severity. The disease develops when muscle tone of the upper airway diminishes during sleep and negative pressures associated with inspiration result in collapse of the upper airway, preventing air movement and resulting in airway obstruction. The sleeping patient inhales more forcibly, thereby, further lowering upper airway pressures and causing further collapse of the upper airway. During this time, substantially no air movement into the chest occurs and the patient becomes progressively more hypoxic and hypercarbic. Both hypoxemia and hypercarbia produce central nervous system stimulation resulting in arousal. Upon arousal, increase in airway muscle tone opens the airway and the patient rapidly inhales and ventilates quickly to correct the abnormal arterial blood gas values. Generally, the arousal is only modest and the patient is not aware of the arousal. Once blood gas parameters have been corrected, the patient begins to sleep more deeply, upper airway tone again diminishes, and the upper airway collapses resulting in sequential and cyclic apneic arousal episodes.
The duration and severity of each apnea is quite variable from patient to patient and with the same patient throughout the night. Indeed, the disease process represents a spectrum of severity from mild snoring, which is associated with incomplete and inconsequential airway obstruction, to severe apneas which can result in fatal hypoxemia.
This disease commonly results in excessive daytime sleepiness and can disrupt cognitive function during the day due to fragmentation of sleep during the night associated with recurrent arousals of which the patient is not aware.
Although this disease commonly affects obese patients, it may occur in patients with any body habitus. Because this disease is so common and because it presents with the subtle and common symptoms of excessive daytime sleepiness, morning headache, and decreasing ability to concentrate during the day, it is critical that an inexpensive technique for accurately diagnosing and treating this disease be developed. Traditionally, this disease has been diagnosed utilizing a complex and expensive multi-channel polysomnogram as the first diagnostic step. This is generally performed in a sleep lab and involves the continuous and simultaneous measurement and recording of an encephalogram, electromyogram, extraoculogram, chest wall plethysmogram, electrocardiogram, measurements of nasal and oral air flow, and pulse oximetry. These, and often other, channels are measured simultaneously throughout the night and these complex recordings are then analyzed to determine the presence or absence of sleep apnea. From the perspective of the primary care physician, the identification of patients with obstructive sleep apnea (OSA) with a primary care practice represents a daunting challenge. This challenge is derived primarily by the fact that OSA represents with extraordinarily common signs and symptoms. These include varying degrees of daytime sleepiness (which may be difficult to distinguish historically from fatigue), inability to concentrate, irritability, headache, and habitual snoring. In addition, the physical findings of OSA are also common and include hypertension, lower extremity swelling, and obesity. Any of these signs may be completely absent and the patient may appear entirely normal to the physician.
There are, therefore, no clear distinguishing historical or physical finds of OSA. In fact, a recent study has shown specificity and sensitivity of clinical evaluation alone to be only 60% and 63%, respectively. The complete lack of sensitivity and specificity of clinical evaluation for the disease is confounded by the fact that it occurs so frequently in specific population groups. In the U.S., OSA is a co-morbin condition in up to 20% of patients with coronary artery disease and 30% of patients with hypertension. The primary care physician, understanding the frequency of this association and the lack of sensitivity of clinical evaluation, is faced with the clear recognition that the majority of patients with OSA within any primary care practice will remain undiagnosed unless a prohibitive number of patients are evaluated by polysomnography.
Since OSA is more common than diabetes and its symptoms are just as subtle, it is critical that all primary care physicians have an inexpensive test which helps assess the pretest probability of the disease. The optimal diagnostic system should easily fall within the budget of the primary care physician for capital expenditures (e.g., $500-$1000) and should be able to be performed within the requirement of complex patient attachments and additional medical technologists, which many primary care physicians would find prohibitive. Indeed, the ideal system would not require complex setup in the home, but rather would require only very simple instructions to the patient. Furthermore, the test should be repeatable, as with a weekend study at no additional cost to the primary care physician, so that multiple consecutive nights could be recorded to assess night-to-night variability in selected patients. In addition, and perhaps most important, patient costs for a diagnostic study should be less than $100 for a single or multiple consecutive night evaluation.
It is also optimal that the home test not be duplicative of conventional polysomnography which, because of its complexity, is best performed in a laboratory setting. On the contrary, the ideal home system would be synergistic with laboratory polysomnography. It is considered advantageous to avoid the use of "mini-polysomnograms" in the home environment. It has been suggested that such conventional multi-channel home systems may actually increase the number of in-lab studies to be performed to "sort out" abnormal home studies (see New Directions for Pulse Oximetry in Sleep Disorders. Mayo Clinic Proceedings, Tobert et al. June 1995; 70:591-92). Furthermore, HMOs and insurance carriers often consider the conventional home multi-channel home studies are done first. Furthermore, in actual practice, such home multi-channel studies often must be repeated or require home setup by a trained medical technologist, which greatly increases the expense.
The issue of sensitivity and specificity with respect to an OSA home diagnostic system must be placed in perspective since the disease is relatively unique in that the "state of disease" is not finite. It is important to recognize that, with OSA there is no clear definition of the lowest threshold of severity which constitutes disease. Indeed, in a recent study, 26% of middle-age adult males met the minimum criteria for OSA based on the apnea/hypopnea index alone. Clearly, a gray area exists between normal and milder forms of airway instability and heightened airway resistance which may or may not require therapy. Indeed, home diagnostic systems which positively identify patients with heightened airway resistance or very mild or normal levels of OSA may simply increase cost through the identification of the vast pool of patients for which further diagnostic evaluation and intervention has no clearly definable indication (again, see New Directions for Pulse Oximetry In Sleep Disorders, Mayo Clinic Proceedings. Tobert et al. June 1995; 70:591-92). The critical challenge is to identify the vast number of undiagnosed patients having OSA of significant severity such that therapeutic intervention has established efficacy. These are the patients most likely to develop the cardiovascular sequelae of OSA.
Furthermore, the increased risk of automobile accidents is directly correlated with severity of disease. The ideal home diagnostic system should be selectively sensitive for disease of significant severity such that treatment has established efficacy.
The problem with this traditional approach of using the sleep lab as a first test for case-finding is that such complex sleep testing costs between one-thousand to thirty five hundred dollars. The available resources for performing the procedure are relatively scarce. Since the patients often perceive the symptoms (e.g. sleepiness) as non-life threatening, many patients find the cost for the evaluation prohibitive, even if the physicians choose to refer the patient. In addition, primary care physicians managing HMOs with limited equipment budgets will be understandably reluctant to sent patients with subtle symptoms or in high risk groups for such an expensive initial evaluation without first having an inexpensive means to better assess the probability of disease, and thereby better focus these limited resources. For these reasons, the majority of patients with OSA in the U.S. remain undiagnosed. Since sleep apnea is so common, the cost of diagnosing obstructive sleep apnea in every patient having this disease in the United States would exceed Ten Billion Dollars. It is critical that a new, inexpensive technique of accurately diagnosing sleep apnea be developed.
Nocturnal oximetry alone has been used as a screening tool to screen patients with symptoms suggestive of sleep apnea to identify whether or not oxygen desaturations of hemoglobin occur. Microprocessors have been used to summarize nocturnal oximetry recordings and to calculate the percentage of time spent below certain values of oxygen saturation However, oxygen desaturation of hemoglobin can be caused by artifact, hypoventilation, ventilation perfusion mismatching. For these reasons, such desaturations identified on nocturnal oximetry are not specific for sleep apnea and the diagnosis of sleep apnea has generally required expensive formal polysomnography.
The present invention comprises a system and technique for deriving and utilizing the analysis of graphical pulse oximetry-derived waveforms as a function of time to accurately diagnose sleep apnea. This invention is therefore synergistic with conventional sleep lab testing since the convention sleep lab can subsequently be used in a more efficient and focused manner to assure mitigation of arousals and sleep fragmentation by therapy.
It is the purpose of this invention to provide an inexpensive system for the collection and analysis of pulse oximetry values as a function of time during sleep to provide a diagnosis of sleep apnea with a high degree of specificity.
This invention provides a reliable and specific means for the diagnosis of obstructive sleep apnea which can be performed in the patient's home without attendance of technical personnel. It is further the purpose of this invention to provide an inexpensive and accurate means to both screen for and specifically diagnose obstructive sleep apnea by a single overnight recording in the patient's home without the need for multiple connections to different parts of the patient's body. It is further the purpose of this invention to define a technique for diagnosing obstructive sleep apnea utilizing the calculation of the ascending and descending slope ratio of phasic oxygen desaturations measured during sleep.
Specifically, the present invention defines a device for diagnosing sleep apnea, that has the following components. First, a means must determine an oxygen saturation of a patient's blood. This saturation value is coupled to a means for identifying a desaturation event based on the saturation value. The desaturation event is one in which said oxygen saturation falls below a baseline level by a predetermined amount and for a predetermined time. The slope of the event is calculated by means for calculating a slope of said desaturation event representing a rate of change per unit time of fall of oxygen saturation. This slope is used by a means for comparing said calculated slope with a value of slope which is determined in advance to be indicative of sleep apnea, and determination of diagnosis of sleep apnea is made based on said comparing.
The comparing can be done by:
1) comparing with an absolute number which is likely to indicate a sleep apnea, or PA1 2). comparing with other slopes taken at different times.
The identifying means can also identify a resaturation, immediately following said desaturation and coupled with said desaturation, in which the oxygen saturation rises, and wherein the determination can also be based on a slope of said resaturation.
Many other ways of calculating the slope are also disclosed herein.
These and other aspects of the invention will now be described in detail with reference to the accompanying drawings, wherein: